This invention relates to cylindrical bias magnet apparatus for use in nuclear magnetic resonance (NMR) applications and more particularly to cylindrical NMR bias magnet apparatus employing permanent magnets and methods therefor.
The many advantages of nuclear magnetic resonance as an imaging tecnhique for medical and biological purposes have become well known. Conventional NMR imaging systems require a bias magnet for generating a strong uniform magnetic field to promote the alignment of the nuclear magnetic moments of muclei in the specimen material parallel and anti-parallel to the applied field with a slight majority of magnetic moments in the anti-parallel position.
When perturbed from equilibrium, the proton magnetic moments (spins) precess at a frequency which is proportional to the applied field. The frequency of precession is called the resonant frequency. The spins are rotated from equilibrium by a radio frequency coil which produces a magnetic field perpendicular to the main field. The R.F. field alternates at the resonant frequency of the spin.
Spacial localization is achieved with gradient coils which are employed to produce a magnetic field which changes with position. Since the sample protons resonate at a frequency which is proportional to the applied magnetic field, the protons are caused to resonate at a frequency which is proportional to position. Thus the spacial position of each spin is determined by its resonant frequency.
An R.F. receiver is used to detect the signal produced by the precessing magnetic moments and a computer system is used to frequency analyze the signal to produce the resulting image. In addition, tissue contrast is produced by detection of the relaxation time associated with the spins return to equilibrium position. Additional tissue contrast is supplied by detection of a signal decay time constant associated with irrecoverable dephasing caused by magnetic fields produced by adjacent spins.
The magnet system employed to generate the bias field in NMR applications is required to generate a substantial magnetic field in a range typically from 1 to 5 kilogauss and this field must be highly uniform throughout the volume inw hich the specimen under test is disposed. Where human specimens are involved and the NMR system is required to accommodate any portion of the specimen, the bias field must typically be developed across a relatively large bore and uniformly extend for a substantial interval.
From the foregoing, it will be seen that magnetic circuits employed for purposes of producing the bias field are subject to rather onerous requirements which have only been achievable through resort to magnetic circuits which are relatively massive and cost intensive. One approach originally finding great favor due to the large fields available was to employ so-called superconducting magnets for the generation of the bias magnetic field. Here, the actual magnetic structure involved was reasonable in size and weight; however, the operating costs associated with the cryogenic equipment and the maintenance costs thereof tended to be expensive. In addition, in some cases at least, the widely fringing field generated thereby proved to be highly disadvantageous if not outright dangerous.
Magnetic circuits employing permanent magnets were more desirable as not subject to the operating deficiencies and maintenance costs associated with cryogenic systems. Here, however, the resulting magnetic circuits tended to be massive often exhibiting characteristics which greatly restricted their location and, in addition, were excessively costly in manufacture due to the large amount of magnetic material involved and the limited field strength available from conventional magnetic materials. Furthermore, even with the massive magnetic structures involved, resort to costly rare earth alloy magnetic materials was often necessary to achieve desired field strength.
Substantial design effort has been devoted to the development of permanent magnet assemblies for use in producing NMR bias fields in sufficiently large bore configurations to accommodate human specimens. Thus, for instance, U.S. Pat. Nos. 4,498,048 and 4,580,098 as issued on Feb. 5, 1985 and Apr. 1, 1986 to Lee et al. and Gluckstern et al., respectively, and assigned to E.I. duPont de Nemours and Company Inc. disclose a permanent magnet bias system which is formed of a plurality of collars or rings wherein each ring takes the form of a dipole magnet made of permanent magnet material. Each ring or collar comprises precisely located segments of permanent magnet material wherein each segment is formed of a large number of permanent magnet bricks made of rare earth alloy, ferrite ceramic material, or the like, formed into a trapozoidal segment. The segments, once formed, are positioned such that the anisotropic magnet axis thereof is arranged in each ring or collar according to the formula a=2.phi. where .phi. is the angle between the radial symmetry line of the segment and the X axis of the dipole ring magnet formed and a is the angle between the anisotropic axis of the segment and the axis. Each segment is magnetized after the segment is formed. The segments are then positioned within a collar or ring so that the entire ring or collar is formed. The segments are tuned by a process of repositioning to eliminate nonuniformities in the dipole magnetic field as determined by a hall effect probe or the like.
In an apparently improved structure, as described in U.S. Pat. No. 4,538,130, as issued on Aug. 27, 1985 to Gluckstern et al. as assigned to Field Effects Inc. of Action, Mass., a ring structure formed of permanent magnets is also disclosed. Here, however, the segments of the ring are formed by four blocks of magnetic material which are already magnetized. The orientation of the anisotropic axis of magnetization of the blocks is such that a dipole ring is again formed with the anisotropic axis of each segment being arranged in the same manner as described in U.S. Pat. Nos. 4,498,048 and 4,580,098. Once the ring is formed tuning for purposes of reducing nonuniformities again occurs. While the structure set forth in U.S. Pat. No. 4,538,130 is substantially simplified over that previously set forth, the machining involved to achieve blocks having the appropriate orientation of the anisotropic axis is highly labor intensive and results in a structure which is quite costly.
A further improvement in permanent magnet bias assemblies for NMR applications is set forth in U.S. Pat. No. 4,639,673 as issued on Jan. 27, 1987 to Heine Zilstra and assigned to U.S. Philips Corporation of New York, New York. Here a ring-type structure is again disclosed wherein permanent magnet segments are disposed about the ring and aligned according to the formula a=2.phi.. However, each segment is disclosed as taking either the form of a holder having a magnetic bar disposed therein or, alternatively, bars of cylindrical-segment shaped cross section may be employed.
The structure set forth in U.S. Pat. No. 4,639,673 is again simplified over its predecessors and has the advantage that in the process of fine tuning to avoid or reduce field nonuniformities associated with discontinuities in the materials, harmonics or the like, the bars of permanent magnetic material can be rotated either in their sleeves or within the housing. However, again, machining of the magnetic material necessitated by this structure results in the cost of the magnetic material being subject to a multiplier of from four to ten compared with the original cost of the material.
While each of the dipole magnetic structures described above provides a magnetic bias assembly capable of large bore configurations suitable for NMR applications and use magnetic material in a relatively efficient manner, the fabrication costs thereof are extremely high due to the relatively complex structures involved, the large amount of machining required and painstaking assembly steps necessary. Furthermore, while each structure is assembled in a manner to achieve high field uniformity according to the formula a=2.phi. and may be subsequently fine tuned to reduce measured discontinuities in the field associated with assembly errors, material discontinuities, harmonics or the like, these structures provide only limited capability of tuning wherein additional harmonics useful in reducing the presence of unwanted harmonics may be introduced. In addition, each of these designs would appear to contain at least an implied preference for extremely costly magnetic materials such as rare earth alloys due to the field requirements thereof and the magnetic circuits constructed. Hence, when machining costs are considered, it is not unusual for the costs associated with the magnetic materials utilized to approach $50 a pound with 5,000 to 10,000 pounds of material being involved.